Stop 2 - aitik cu-au-ag mine Roger Nordin Boliden Mineral AB, Boliden, Sweden Christina Wanhainen Luleå University of Technology, Luleå, Sweden Riikka Aaltonen Boliden Mineral AB, Boliden, Sweden The Aitik Cu-Au-Ag mine is situated in Norrbotten County, northern Sweden, some 100 km north of the Arctic Circle and 17 km east of Gällivare town (Fig. 1). The mine started operating in 1968 at a capacity of 2 Mt of ore annually. Subsequent expansions to 5 Mt (1970 72), 11 Mt (1979 81), have brought the capacity up to 16 Mt (1989 91). The next expansion will be operational in 2010 2011 figure 1. Geology of the Gällivare-Aitik area; from Martinsson and Wanhainen (2000). 78
Stefan Bergman and will bring the capacity up to 33 Mt of ore in 2010, which will be ramped up to 36 Mt annually. The Aitik mine (Figs. 2 and 3) is a conventional large open-pit operation with an in-pit crusher (18.4 Mt of ore mined 2006). The Cu-Au-Ag ore is moved by trucks carrying 240 tonnes of ore to the crushers. The ore is crushed, milled and processed in the flotation plant yielding a chalcopyrite concentrate. The economic product is a Cu-(Au-Ag) concentrate with an average grade of 27 29 % Cu, 8 ppm Au and 250 ppm Ag. The concentrate is transported by truck to Gällivare and then railed 400 km to the Rönnskär Cu smelter east of Skellefteå, where LME (London Metal Exchange) grade Cu cathodes are produced. By-product gold and silver are also extracted at Rönnskär to produce metallic Au and Ag. Sulphur is captured by the smelter and converted into sulphuric acid. In 2006, Aitik produced about 29 % of the required feed of the Rönnskär smelter, or 240,000 tonnes of Cu concentrate. An average year at Aitik would yield some 60,000 tonnes of Cu-in-concentrate, 1.5 2 tonnes of Au, and some 40 50 tonnes of Ag, from 17 18 Mt of ore. Since the start of mining at Aitik in 1968, approximately 450 Mt of ore have been mined from a 3 km long, 1 km wide and 390 m deep open pit. In addition, some 400 Mt of waste rocks have been removed to expose the ore body. Proven and probable ore reserves at the start of 2007 were 625 Mt with 0.28 % Cu, 0.2 ppm Au and 2 ppm Ag. Additional measured and indicated mineral resources were 858 Mt with 0.24 % Cu, 0.2 ppm Au and 2 ppm Ag, with an additional 66 Mt of inferred resources grading 0.25 % Cu, 0.2 ppm Au and 2 ppm Ag (Boliden AB 2006). This makes Aitik the largest Cu deposit in the Fennoscandian Shield and one of the largest Au-rich porphyry copper deposits in the world. The current mine life, including the expansion to up to 36 Mt/a, will allow the mine to continue to operate until 2026. The final dimensions of the open pit in 2026 will be 5000 m long by 1400 m wide and 600 m deep. Exploration in the area is ongoing. figure 2. Local geology and excursion stop at the Aitik mine. Geology from Wanhainen and Martinsson (1999). 79
figure 3. Metal distribution at Aitik for copper (A) and gold (B) for the 100, 300 and 500 m horizontal levels. Class limits are chosen after the classification of mineable to waste rock and low- to high-grade ore used by Boliden AB. From Wanhainen et al. (2003b). Safety rules - important note: The instruction of your guides MUST be followed at all times. Pay special attention to the movement of the very large machinery. If you are taking samples, make sure that the location(s) are safe. A hard hat must be worn at all times. The local mine geology at Aitik (Figs. 2 and 4) is divided into 3 main parts, i.e. the hanging wall, main ore zone and the footwall complex. The hanging wall is basically one unit of strongly banded hornblende gneisses. The main ore zone consists of three main units, a muscovite schist, biotite schist and biotite gneisses. These rocks are strongly deformed and altered which obscure their primary character. However, their chemical character suggests a magmatic precursor of intermediate composition and, based on the knowledge from areas outside the mine, a volcaniclastic origin (Wanhainen & Martinsson 1999). The most important footwall unit is the quartz monzodioritic intrusive. Other intrusives of interest are the pegmatite dykes which cross cut the hanging wall, main ore zone and the footwall complex. The main ore zone dips roughly 45 to the west (Fig. 4), and the lower ore contact consists of a gradational weakening of the copper grade at roughly 50 to the west. The lower contact is approximately where biotite gneisses change into regional biotiteamphibole gneiss. Sporadic Cu mineralisation of no economic interest exists in these footwall gneisses. The footwall quartz monzodiorite in the southern part of the mine is part of newly started series of push backs. Below follows a detailed description of the Aitik mine rock units (see also the rock types depicted in Fig. 5): 80
Stefan Bergman figure 4. Section across the Aitik deposit, view to the north, 200 m grid. hornblende-banded gneiss is a finely banded unit (mm to cm wide bands) with alternating dark olive green and light grey layers. This unit is more than 250 m thick, and overlies the main ore zone. It is devoid of sulphides. Mineralogically it is dominated by hornblende, with biotite, quartz and minor plagioclase. The light grey bands have weak to moderate sericitic and chloritic alteration. The unit also shows a red-green microcline-epidote-alteration. Scapolite porhyroblasts of 1 5 mm in diameter occur throughout the unit. Other accessory minerals are magnetite and tourmaline. The fine-grained unit likely represents original compositional variations, even though it is strongly metamorphosed. Based on field evidence, it is suggested that the fining upwards of the layering shows that way-up is towards the west. The unit appears to have been tectonically emplaced over the main ore zone. The fault at the contact appears to be a thrust. The boundary between the main ore zone and the hornblendebanded gneiss is in places highly fractured, causing problems for drilling. The border zone between hornblende banded gneisses and the main ore zone is also intruded by several pegmatite dykes up to 40 m wide. Quartz-muscovite (sericite) schist constitutes the upper part of the main ore zone. The unit is roughly 200 m thick, and consist of a strongly foliated muscovite-rich matrix with quartz, biotite, microcline and plagioclase. It is a light-buff coloured unit showing a sharp contact with the overlying hornblende-banded gneiss and a gradational lower contact grading into biotite schist. Accessory minerals in this unit are epidote, tourmaline, magnetite and garnet. Magnetite occurs as occasional mm-scale porphyroblasts, and also as fine dissemination (1 3 % magnetite). The sulphide minerals are dominated by pyrite and chalcopyrite (py > cpy > po). Total sulphur content can reach 5 7 %, corresponding to 15 20 vol-% of sulphides. The muscovite schist has a Cpy:Py ratio ranging from 1:2 to 1:7. The upper contact of the muscovite schist contains a sulphide rich zone, 5 40 m wide with up to 20 25 % sulphides. Gold and copper zonation is shown in Figure 3. Gold and copper grades increase at depth in the northern part of the pit. Pyrrhotite and molybdenite occur as less common sulphides. Pyrite typically occurs as large blebs, or along foliation planes, and as small veinlets. The Ba content of the unit is quite high, in the order of 1,000 several 1,000s of ppm. biotite schist constitutes the middle part of the main ore zone. It is gradational into the biotite gneisses below as well as to the muscovite schists above. The thickness is on average 150 m. This unit is strongly foliated and sheared in a roughly northsouth direction. It contains pyrite and chalcopyrite dissemination and veinlets, and chalcopyrite clots, 81
with pyrite and chalcopyrite as equal volumes. Magnetite occurs as a fine dissemination with grains commonly enclosed within amphibole and/or garnet porphyroblasts. Molybdenite is present in the northern part of the mineralisation. Biotite dominates over muscovite, and defines a strong foliation. Thin veinlets of quartz, commonly deformed, occur in this unit. Undeformed veinlets with late zeolites and epidote occasionally occur within the unit. biotite gneisses constitute the lowermost part of the main ore zone, although the rock type is not always present. They commonly display zones of red garnet (spessartine-almandine) and more gneissic, coarser-grained character than the strongly foliated biotite schist. Mineralisation is of the same style as in the biotite schist. Quartz monzodiorite is the dominant footwall unit, being up to 600 m thick. It shows mediumgrained equigranular, 2 5 mm phases as well as strongly porphyritic phases. Transition between these phases (= subphases of the quartz monzodiorite) is almost always gradational. The quartz monzodiorite contains plagioclase phenocrysts being up to 7 9 mm in size. The plagioclase show compositional zoning. The matrix of the quartz monzodiorite consists of a fine-grained mixture of plagioclase, quartz, biotite and minor sericite. Alteration is commonly present as weak silicification and pinkish potassic alteration. Mineralisation is dominated by fracture-controlled py-cpy±mos 2, but finely disseminated sulphides are also present. A minor accessory mineral is epidote, which can contain fine grained cpy. Hornblende and quartztourmaline veinlets occur throughout this unit. Veining of quartz, quartz-tourmaline, gypsum, gypsum-fluorite and zeolites occur as mm cm wide veinlets. The zeolites present are stilbite, chabazite and desmine, and calcite and baryte have also been observed in this association. These stockwork veins cut each other at high angles, but zones of deformation are also present. The quartz monzodiorite has a zircon U-Pb age of ca. 1.89 Ga (Wanhainen et al. 2006), which fits well with reported ages for regional Haparanda suite granitoids (Bergman et al. 2001). feldspar-porphyritic andesitic intrusives occur as large dykes and occasionally show chilled margins. These types of intrusives occur throughout the entire stratigraphic column, but are more common in the footwall area. These dykes are strongly porphyritic in character, with large feldspar phenocryst laths, up to 25 mm long and 4 5 mm wide. They are set in a dark olive green matrix of hornblende, biotite, chlorite, and occasionally actinolite or tremolite. The fine-grained, equigranular variety of this rock is termed amphibolite in the mine. Sulphides, when present, are typically pyrite-chalcopyrite at a 1:1 ratio, and they appear to be both remobilised from the adjacent rocks and to be present within the feldspar porphyritic unit. amphibole and amphibole-biotite gneisses constitute a major part of the footwall unit. These rocks typically exhibit an anastomosing weak network of 5 30 mm wide hornblende veinlets or schlieren with a light-coloured feldspar (albite) rim. Biotite defines a weak foliation, and porphyroblastic garnet is commonly present forming 1 5 vol-% of the rock. Sporadic scapolite is present as small grains and as zones of intense scapolitisation. Magnetite is a common accessory (1 3 %), and occurs as small porphyroblasts and as veinlets. Thin pegmatite dykes are common; they may reach a maximum width of 40 m. Their distribution is varied within the mine area with the largest frequency of the dykes in and around the hanging wall contact, where they are unmineralised. At the hanging wall contact, they are oriented roughly N-S and dip about 60º to the west. In the main ore zone, the pegmatite dykes occur less frequently, and one series of the dykes show a NNW orientation and a steep dip. The pegmatites commonly have been intruded forcefully since they can contain large fragments of the adjacent country rock. When they intrude mineralised host rock they can also exhibit py-cpy mineralisation. Mineralogically they are dominated by very-coarse grained microcline, quartz and typically long prismatic black tourmaline. Greenish muscovite flakes also are common. Accessory minerals within the pegmatites are molybdenite and fluorite. 82
Stefan Bergman figure 5. Rock types at Aitik. A. Hornblende-banded gneiss AIA1026 (HWC at 19.40 m). Finely banded unit with minor coarser bands. B. Muscovite schist AIA1042 (MOZ) at 151.50 m. Displaying mixed nature with alternating muscovite and biotite-bands. C. Biotite schist AIA1042 (MOZ 133.95 m) Dark grey rock with biotite bands, displaying some muscovite. Garnet-porphyroblasts and dissemination of chalcopyrite, pyrite and pyrrhotite. D. Amphibole-biotite-gneiss AIA1021 (MOZ at 112.20 m). Metamorphic hornblende patches and schlieren. E. Diorite AIA1042 (MOZ/FWC). Coarse-grained and porphyritic. Overprinting metamorphic amphibole alteration and silicification causes diffuse textures. F. Diorite porphyritic with altered plagioclase phenocrysts (potassic alteration) AIA1042 (at 505.30 m) Dioritic matrix. Potassic alteration and late gypsum veinlets. G. Mineralised diorite. H. Feldspar porphyritic gabbro AIA1042 (stratigraphic footwall at 609.40 m). Andesitic matrix. Plagioclase laths (5-15 mm). 83
Genetic model The Aitik host rocks belong to the regionally widespread Haparanda suite of intrusions and Porphyrite group of comagmatic volcanic rocks (Wanhainen & Martinsson 1999, Wanhainen et al. 2006) which were generated during subduction of oceanic crust beneath the Archaean craton around 1.9 Ga, during the Svecokarelian orogeny (Weihed 2003). High-salinity fluids (30 38 eq. wt. % NaCl+CaCl 2 ) responsible for chalcopyrite-pyrite mineralisation in Aitik were released contemporaneously with quartz monzodiorite emplacement and quartz stockwork formation at ca. 1.89 Ga and caused potassic alteration of the intrusive and surrounding volcaniclastic rocks. The mineralised quartz monzodiorite in the footwall is suggested to represent an apophyse from a larger intrusion at depth consistent with the porphyry copper model presented by Lowell and Guilbert (1970). Furthermore, zonation and alteration patterns, although disturbed, fit quite well with this model (Yngström et al. 1986, Monro 1988, Wanhainen 2005). However, all features of the main ore zone are not typical for a porphyry system, and Aitik is suggested to be hybrid in character with an affinity to both IOCG and porphyry-copper mineralisation based on the character of the high salinity ore fluids, the alteration and mineralisation styles, and on the 160 Ma (Re-Os molybdenite and U-P titanite and zircon dating) evolution of the deposit (Wanhainen et al. 2003a, Wanhainen et al. 2005, Wanhainen et al. 2006), including a regional mineralising event of IOCG-type at ca. 1.8 Ga. 84